1. Field of the Invention
In general this particular invention relates to the hot-forging of metal articles to a predetermined shape. More specifically, however, the present invention relates to a novel and improved method in forging apparatus for forging metal articles having an inner bore of small diameter and the means for keeping thermally and dimensionally stable the inner core rod of the forging apparatus which forms the small bore in the metal article during hot forging.
2. Brief Description of the Prior Art
Present day forging methods and apparatus include the use of a solid core rod for forming the bore of generally concentric, cylindrical metal articles such as bushings, roller bearing race members, spur gears and the like. Typically such an apparatus will include a die for forming the axially extending external surfaces of a powdered metal part, an upper and lower punch for forming the end surfaces of such a part and the cylindrical core rod concentrically arranged and in sliding contact with one of the punches for forming the bore of the part. The die and core rod are generally stationary while the lower punch acts as a knockout mechanism at the conclusion of the forging stroke and the upper punch is mounted in a hydraulically or mechanically actuated ram and is that part of forging apparatus which moves the metal during forging stroke. The upper punch normally includes a bore which receives the uppermost portion of the core rod during the lower end of the forging stroke. Such an apparatus is shown in U.S. Pat. No. 3,761,257, assigned to the assignee of the subject invention, and it will be noted that the inner core rod is solid and no means is provided for cooling. Such an apparatus is perfectly suited for the forging of parts having a relatively large diameter inner bore in the order of four inches or more. Such a core rod is generally large enough that it will satisfactorily dissipate any heat build up at the die cavity caused during the forging stroke. Thermal stability, and consequently physical or dimensional stability of the core rod, will thus be maintained. However such a forging apparatus has heretofore been unsatisfactory for the hot-forging of parts having a bore of smaller diameter, particularly, those of 11/2 inch or less. With such a small bore, the amount of heat absorbed by the proportion of the small core rod causes the core rod to become thermally and dimensionally unstable. The high temperatures cause thermal stress and thus premature failure. Likewise the increased thermal expansion of the core rod provides less control over the dimensional characteristics of the bore of the part being forged. Consequently, for the hot forging of parts having a bore diameter ranging from about four inches down to 11/2 inch it has been fairly common to continously circulate a heat transfer fluid through the mandrel, in such the same way that the die itself is maintained at a stable temperature. Typically this means maintaining the heat transfer fluid at a temperature ranging from about 150.degree. F. for the smaller bore parts to about 450.degree. F. for the larger bore parts, circulating it through the mandrel at a rate sufficient to carry away the heat and then cooling it back down through a heat exchanger to its initial incoming temperature. Similarly it is known to cool the core rod in a manner as shown in U.S. Pat. No. 2,950,816; however such an apparatus and the technique practiced with such apparatus is considered undesirable for this particular application since it teaches the alternate circulation of water and air through the mandrel to maintain it at a preselected and rather wide temperature range of 480.degree. F.-1100.degree. F. Neither of these prior art techniques are considered acceptable for maintaining temperatures in the core rod when forging parts having a bore of about 11/2 inches or less, particularly those of 3/4 inch or less, and it is to the forging of these parts at high speed production rates of about 500 pieces per hour that the present invention is directed.